The purpose of the proposed research is 1) to study the interaction and biological role of immunoglobulins cytophilic for lymphocytes and 2) to analyze the structure and search for a function of IgD. The proposal consists of three related projects: 1. a. A receptor specific for the Fc fragment of IgE has recently been demonstrated in our laboratory on subpopulations of human and rat lymphocytes. It is proposed to determine the types of lymphocytes that have IgE receptors and the number of lymphocytes bearing IgE in peripheral blood of normal persons and in patients with different diseases. Isolation of the IgE receptor from cultured lymphoid cell lines will be attempted. Rat lymphocyte functions will be tested in vitro in order to search for the biological function of IgE-bearing lymphocytes. 1. b. The interaction of IgG subclasses with K lymphocytes will be studied. Particular emphasis will be placed on IgG2 and 4 subclasses which appear to enhance cytotoxicity. It will be determined if the enhancement is the result of a modulation of K lymphocytes or a reaction with other lymphocytes and/or monocytes. The number of Fc receptors and their affinity to IgG will be studied in normal and stimulated lymphocyte populations. 2. Present studies of the amino acid sequence of delta chains of IgD myeloma proteins will be continued. Cell-bound IgD will be isolated and certain structures of it compared with those of myeloma IgD. The effect of purified F(ab)2 anti-IgD on primary and secondary immune responses of monkey spleen cells to sheep erythrocytes in vitro will be studied in order to evaluate a possible role of cell-bound IgD in lymphocyte differentiation.